Changing ArcGIS portal DNS alias post installation -

I have successfully installed and configured ArcGIS 10.6 (single site deployment) ArcGIS server federated with Portal. This is almost brandnew portal. Now business team wants change in application url.i.e. DNS alias needs to be changed.

You can lock down your web server so that only client requests to your portal go through the DNS alias. Your backend config can then continue with hostname, e.g. web adaptor config, federation url's. In my opinion, you should not need to change anything besides stopping access via the hostname url. I have built Enterprise GIS' and had IT provide a DNS alias for all machines weeks after the fact with no issues / requirements to change any config on my end.

Good to know. So during installation you simply used the hostnames where required e.g. URL's for federating ArcGIS Server with Portal? Once you had the DNS alias' created you didnt alter any config like WebContextURL?

I was prepared to edit the WebContextURL, but did not do so by default. I attributed this to the hardiness of the esri stack and/or fluke.

All URL's were FQDN during the install & configuration. The only change in this space is that the Portal Web Adaptor URL was configured to the DNS after installation & configuration. In portaladmin/system/webadaptors/<ID>/edit

I'm surprised you didn't need to update the WebContextURL as Finnian O'Connor mentioned. Portal will want to redirect to the internal machine name or privatePortalURL if the incoming request isn't from the WebContextURL.